Abstract/Summary

In July 2021 a cut-off low-pressure system brought extreme rainfall to Western Europe, leading to flooding that caused loss of life and infrastructure damage. Here, we use ensemble boosting to investigate alternative storylines of the event, given the observed dynamical situation. Using a fully-coupled free-running climate model we identify atmospheric flow analogues of the 2021 event in an initial-condition large ensemble. These analogues are re-initialized with slightly perturbed atmospheric initial conditions to generate physically plausible alternative storylines. These storylines are used to investigate how a potentially worse event could have unfolded given the same large-scale dynamics. We identify rainfall events with longer persistence and larger extent, yet the observed event appears to be towards the upper end of what is plausible in the current climate. Such storylines can be used to prepare for possible future events, helping society to imagine dangerous, but plausible, scenarios.